1. Field of the Invention
This invention relates to an evaluation method for a polycrystalline silicon film formed using a low pressure chemical vapor deposition (LPCVD) method or a like method.
2. Description of the Related Art
Conventionally, since a simple method for a film quality of a polycrystalline silicon film has not been available, in-line evaluation of a film quality of a polycrystalline silicon film has not been performed in a manufacturing process of semiconductor devices. However, it is widely known that the film quality of a polycrystalline silicon film has a significant influence on the characteristics of a semiconductor device. For example, in a manufacturing process of a bipolar transistor, a polycrystalline silicon film, for example, of the n type is formed on a base region and heat treatment is performed to form an n-type emitter diffusion layer. Since difference in particle size of polycrystalline silicon, interface condition with a substrate and so forth makes the diffusion rate of an impurity different, the emitter diffusion layer is formed in different junction depth and consequently in different base width (height).
Conventionally, a device is produced and a characteristic of the device is evaluated to indirectly effect evaluation of a polycrystalline silicon film which has been used to produce the device. This evaluation method is described with reference to FIGS. 4(a) to 4(e) taking a bipolar transistor as an example.
First, as seen in FIG. 4(a), an n.sup.+ -type buried layer 12 is formed on a p-type silicon substrate 11 applying a photo-lithography method, a thermal diffusion method and so forth, and an n-type epitaxial layer 13 is grown using a reduced pressure epitaxial growth method or the like. Then, as shown in FIG. 4(b), a base diffused layer 14 is formed applying a photo-lithography method and an ion implantation method or the like, and similarly, a collector lead diffused layer 15 is formed applying a photo-lithography method and an ion implantation method or the like similarly, whereafter a silicon oxide film 16 is grown on the n-type epitaxial layer 13, base diffused layer 14 and collector lead diffused layer 15 using a CVD method or the like. Then, in an emitter formation region, an emitter opening 17 is formed using a photo-lithography method and a dry or wet etching method. Thereafter, as shown in FIG. 4(c), a polycrystalline silicon film 18 is grown using an LPCVD method, and an n-type impurity such as, for example, arsenic (As) is ion implanted over the entire area of the polycrystalline silicon film 18.
Then, as shown in FIG. 4(d), the polycrystalline silicon film 18 is patterned so as to cover over the emitter opening 17 using a photo-lithography method and a dry etching method, and in order to isolate a transistor and wiring lines, an inter-layer insulating film 19 in the form of a silicon oxide film or the like is grown on the polycrystalline silicon film 18 using a CVD method. Thereafter, heat treatment is performed around 900.degree. C. to diffuse As in the polycrystalline silicon film 18 in the emitter opening 17 into the base diffused layer 14 to form an emitter diffused layer 20. Finally, as shown in FIG. 4(e), contact openings for electrodes are formed above the base diffused layer 14, emitter diffused layer 20 and collector lead diffused layer 15 using a photo-lithography method and a dry or wet etching method, and a layer of an electrode material such as aluminum is formed by a sputtering method and then patterned using a photo-lithography method and a dry etching method to form a base electrode 21, an emitter electrode 22 and a collector 23.
Then, the emitter electrode 22 is connected to the ground potential while the base electrode 21 and the collector 23 are connected to a positive potential, and a value (current amplification factor=hFE) is calculated by dividing a current flowing through the collector electrode 23 by a current flowing through the base electrode 21 then to measure a characteristic of the transistor. Here, if the value hFE is within a predetermined range, then it is determined that the polycrystalline silicon film is formed in a normal state. In any other case, it is estimated that the film quality of the polycrystalline silicon film is defective, and evaluation of the film quality is performed and a countermeasure is taken. For the evaluation of the film quality, a transmission electron microscope (TEM) is used in most cases. In particular, a polycrystalline silicon film is cut into a thin piece, and the thin piece is mounted in position on a transmission electron microscope to observe the crystal structure of it.
Where the method wherein a device is produced and a characteristic of the device is measured to evaluate a polycrystalline silicon film is employed, since all steps for manufacture of the device must be completed, much time is required until evaluation of the film quality becomes possible. Further, since the characteristic of the device is varied also by a factor other than the film quality of the polycrystalline silicon, accurate evaluation is difficult with the evaluation method.
Further, where a transmission electron microscope is used for evaluation of the film quality of a polycrystalline silicon film, it is very cumbersome and requires many man-hours to produce a specimen for the evaluation, and much time is required until observation for particle sizes becomes possible. Furthermore, a correlation between a result of observation and an actual device characteristic cannot be discriminated clearly, and besides, integrated evaluation of polycrystalline silicon including an interface condition between the polycrystalline silicon and a silicon substrate cannot be performed.